1. Field of the Invention
This invention relates to audio speakers and in particular to the construction of audio speakers that have minimization of wobble of the voice coil bobbin and all moving parts during operation, minimization of damage to the voice coil and bobbin when the speaker is over driven and cooling of the voice coil and bobbin during operation.
2. Description of the Related Art
A goal of sound reproduction equipment is to provide a life-like sound quality to the listener. Life-like sound quality is understood to be best achieved when a sound system, including the speakers, have a flat frequency response curve throughout the range of sound frequencies audible to the human ear, generally 20 to 20,000 Hz. A normal speaker cabinet has an electro magnetically driven speaker cone sealed to an opening in the wall of a sealed, port vented, and passive tuned cabinet.
As TV technology evolved into thin cabinet designs, this pressured speaker technology to do the same hence the need for shallow speaker systems. The need for shallow, low profile speakers are not limited to meeting the home audio demand. Such low profile speakers also have application in cars, boats, airplanes and other locations that will benefit from the depth reduction without taxing the sound pressure level. In cars for example, the available mounting depth behind the door panel is much less than the minimum height of conventional speakers. In order to use conventional speakers in such locations, it is nearly always necessary to use a raised grill cover over the speaker since it necessary to have a portion of the speaker height extend above the surface of the door panel into the passenger compartment.
In addition to mounting depth reduction, the low profile speaker of the present invention offers weight reduction as well. With energy costs on the rise, this comes in handy to reduce operating cost while the speaker is in motion. A typical design of an 8 Inch woofer with conventional technology shows a weight of 18 lbs. while a low profile woofer of the same size could weigh about 6 lbs. This weight benefit is substantial when the woofer size is greater such as 15 inch. A 15 inch low profile speaker of the present invention weighs about 10 lbs. with a mounting depth of 3 inches while a conventional 15 inch speaker weighs about 30 to 40 lbs. with a mounting depth of 8 to 9 inches. Carrying such a dead weight around in a motor vehicle simply increases gas guzzling.
For the most part, subwoofer construction has followed conventional technology—the use of an oscillating diaphragm that responds to a varying magnetic field developed by an applied audio signal. That varying magnetic field causes the diaphragm to be attracted and repelled to and from the intermediate position where the diaphragm rests when no audio signal is applied to the speaker. For the most part, current speaker technology uses a loudspeaker made of a rigid diaphragm, or “cone”, suspended within a speaker frame, or “basket” around the outer edge with a flexible membrane, or “surround”. This membrane allows the cone to move inward and outward when driven by a varying magnetic field resulting from the application of an audio, or “music”, signal applied to the speaker.
Over the years speakers have been designed with a conventional structure—a cone connected to the outer part to a speaker frame, or basket, through a flexible membrane (surround). To develop a back-pressure wave and to control axial movement of the cone, designers installed a secondary part called a “spider” that also connects the inner part of the cone to the speaker frame. Almost all spider materials used are made of cloth that has been treated and pressed in a heated die to form the shape of the spider that was sought. Conventional speakers require a huge mounting depth that render them useless in shallow spaces where consumers now wish to place speakers. For example, a conventional 10 inch diameter speaker, with an excursion of ±1 inch requires a mounting depth of at least 7 inches. Moreover 12 inch diameter conventional speakers requires a mounting depth of at least 7 to 8 inches. Hence conventional speakers clearly will not fit in shallow spaces, such as walls where the mounting depth is limited to about 3.5 inches, or less, unless a smaller diameter conventional speaker is used. Thus, consumer demand has created a need that conventional speakers can not meet and still provide the performance desired by the consumer. Therefore there is a need to develop loudspeakers that have a large piston area with a minimum mounting depth. Low profile speakers designed using the present invention meet that need.
Conventional speakers have many weaknesses that have become much more evident in longer stroke woofers. Since conventional speakers rely upon the glue ring connection of the cone with the voice coil bobbin and spider, that connection is subjected to bending moments that collapse the glue ring during downward (inner stroke movements) and flare outward the glue ring during outward strokes. Additionally, the structure of conventional speakers promotes harmonically related bending of the cone during inward/outward strokes that fatigues the inner portion of the cone and leads into what is known as a neck-cone failure. This typically, partially or completely, breaks the cone into two cones around the neck area. Prior to that type of failure the cone is known to have a cycle of life during which the cone is breaking down and during the slow breakdown of the cone, distortion increases that becomes increasingly unpleasant for the listener. Further conventional speakers have not been designed to maintain the inner suspension (spider) parallel to the outer suspension (surround) as the cone is driven by the voice coil. The spider and surround are each rigidly connected to the inner and outer edges of the cone, respectively, and any misalignment of those connections and/or variations in the material of the spider, surround and cone around the speaker cause the cone to twist in opposite directions as it is driven inward and outward, with the amount of that twisting increasing as the stroke of the voice coil bobbin increases in each direction. This connection configuration can only compromise such a structure as the cone bends as it moves and causes twisting or spiraling movement.
Another problem that results in reduced audio performance of conventional speakers is wobble of the voice coil during operation of the speaker. Current speaker design structures suffer from several compromising parts that play a major role in producing a high level of harmonic distortion. As it has been a trend in speaker design to get the most output from a speaker opening, they resort to increasing the excursion in order to increase the amount air displacement. What previously was a 0.3 inch high voice coil are now a 1.5 inch, and as high as 2 inch, winding height of the voice coil. These increased height voice coils thus move in excess of 1 inch each way, inward and outward. Often speakers can be found where the movement is as much as 1.5 inches each way. During extreme excursions, these woofers are pushed by these long voice coils that weigh three times as much as in previous designs. The motor (voice coil) is connected to the cone and the spider in what is known as the inner suspension.
The cone is a stiff component relative to the suspension and surround, extending outward (generally) and connects the inner suspension to an outer larger diameter suspension. The combination of spider, cone, outer surround, and voice coil bobbin are interconnected to oscillate axially. When an audio signal with a frequency F is sent to the voice coil it develops a variable magnetic field that interacts with the fixed magnetic field produced by the magnet assembly to produce an oscillating force. During these oscillations, the moving parts are subjected to a uniform internal pressure due to the compressed air in the enclosure and tension developed by the spider and surround. The spider and surround each have some manufacturing offset that tend to be apparent during long strokes as the moving elements will start to wobble. The cone typically is made of processed materials (e.g., pressed paper) thus the cone also possesses a non linear stiffness that leads to another offset. The combination of these offsets leads to wobble of the voice coil bobbin within the air gap of the magnet assembly.
That wobble can distort the sound produced in varying degrees as the voice coil travels inward and outward in many ways, e.g., distorting the shape of the cone. Wobble can also reduce the useful life of a speaker by repeatedly over stressing the cone and other components that eventually results in failure of the component, e.g., a crack or a tear in the cone, partial separation of the cone and surround, etc. Wobble can even result in total failure of the speaker. This can occur if the voice coil is over driven outward with the lower edge of the voice coil bobbin coming completely out of the air gap of the magnet assembly with the wobble shifting the lower edge of the voice coil bobbin so that it is no longer aligned with the slot in the magnet assembly. The bottom edge of the voice coil bobbin then hangs up on the top of the magnet assembly as the tension in the spider and surround pull the cone and attached voice coil bobbin downward when the lower end of the voice coil bobbin does not reenter the magnet assembly. Once hung up on the top of the magnet assembly the speaker can no longer move regardless of whatever drive signal is applied to the voice coil since the voice coil is no longer in the magnetic field of the magnet assembly so the drive signal does not interact with the magnetic field, i.e., no signal when applied to the voice coil will be able to move the voice coil bobbin.
Another problem that has occurred when a speaker has been over driven in a downward stroke is the bobbin, and possibly the voice coil as well, being damaged when the bottom edge of the bobbin strikes the bottom of the air gap in the magnet assembly. When this occurs, several different things may occur. The bobbin bottom edge can be bent so that the bobbin scrapes the interior of the air gap on the up stroke, the bobbin can be bent out of round which can not only cause scraping but also create serious wobble of the bobbin. Any impeding of the movement of the bobbin may cause distortion in the sound produced by the speaker as well as overheating of the voice coil which can produce total failure of the speaker. The bobbin striking the bottom of the air gap can also cause compression of some of the windings of the voice coil that will distort the sound produced by the speaker, and could even break one or more windings of the voice coil which will cause total failure of the speaker.
In a conventional speaker, the cone is suspended by an outer surround and an inner surround. The outer surround is typically connected to the largest diameter of the cone. The inner surround is connected to smallest ring of the cone. This means that the spider (typically used on the inner surround) will have an inner contact ring with the cone of a typical 2 inch diameter (on a 10 inch woofer, and no larger than 4 inch on a 12 and 15 inch woofer). These inner rings are suppose to control lateral movement so that the cone/voice coil assembly does not wobble. Since the guiding ring in these conventional speakers (spider contact ring with the cone) are small rings, the wobble is a serious reason for failures. Over 20% of woofer failures is attributed to wobble that led the voice coil to rub on the pole piece (the Tyoke shaped metal piece). In analyzing these failures, the voice coil tends to wobble and rub on the Tyoke until it comes apart. Often these failures show that the voice coil jumped on top of the Tyoke. Designers have resorted to the use of 2 spiders (inner and outer) to reduce these wobbles.